Fertiliser in container for plant, timber (6)
I believe the answer is:
potash
'fertiliser' is the definition.
(I've seen this before)
'container for plant timber' is the wordplay.
'container for plant' becomes 'pot'.
'timber' becomes 'ash' (I've seen this before).
'pot'+'ash'='POTASH'
'in' is the link.
(Other definitions for potash that I've seen before include "Alkaline compound" , "Compound used in agriculture and industry - shop at (anag)" , "field dressing" , "Alkali used as fertiliser" , "chemical compound" .)